The influence of seed size and shape on their removal by water erosion

Under semiarid environments where surface runoff takes place, the role of water erosion in the removal of seeds and then in the plant establishment and distribution is unknown. Within the scientific literature, data about the size and shape of seeds exist, but little information can be found about the susceptibility of seeds to be removed by water erosion processes. These data are important not only from an eco-geomorphological point of view, but also for afforestation strategies in order to improve the selection of species to develop the recovery of vegetation on degraded environments. Eighty-three seed species were selected from Southeastern Spain. Variables related to size, such as weight (M), length (L), width (W) and height (H) of these seeds, were measured. From this information, other variables such as the surface (S), volume (V), density (D) and the ratio S/M were calculated. Seed shape was characterised by the Flatness Index (F.I.=L+W/2H) and the Eccentricity Index (E.I.=L/W). Laboratory rainfall simulation experiments at 55 mm h⁻¹ during 25 min on 26×26 cm plots with slope angle equal to 11° were performed with five lots of 50 seeds of each species. Results demonstrated that size is the main factor explaining seed removal, whereas the shape becomes important only when the seeds are larger than 50 mg. Seeds for vegetation recovery planning should have a mass between 10 and 50 mg to avoid removal by water erosion.